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SSI - Solid State Imaging
SSI
Solid State Imaging
Michael J. S. Belton, Team Leader
National Optical Astronomy Observatories
36K
MEASUREMENT OBJECTIVES
- Map the Galilean satellites at a spatial resolution of less than 1 km.
- Investigate the Jovian atmosphere and cloud structure.
- Investigate Jovian atmospheric dynamics.
- Determine the shapes and sizes of the Galilean satellites and determine their spin axis
within +/- 3 km.
- Map the Jovian ring structure and look for associated small satellites.
- Map and characterize auroral phenomena in the Jovian atmosphere and on the
satellites.
- Characterize the state of the atmosphere at the time and place of the Probe
descent.
SUMMARY
The scientific objectives of the solid-state imaging (SSI) camera
investigations have a wide scope: a comparative study of satellite surfaces, a study
of the Jovian atmosphere, characterization of Jovian and satellite
auroral phenomena, and an assessment of the rings of Jupiter. For
the Galilean satellites Io, Europa, Ganymede, and Callisto, the imaging
investigators hope to map a large portion of each surface to a
resolution of 1 kilometer or better. In a few areas, features smaller
than 10 meters will be distinguished. In addition, variations in
color and albedo (reflectivity) will be mapped at a scale of about 2
kilometers. Scientists will look for changes on the surfaces since Voyager.
The shape and the location of the spin
axis of each Galilean satellite will also be measured.
The other smaller satellites will be studied throughout the orbital
tour. Studies will also be made of Jupiter's rings. Small, new
satellites may be found in or near the rings.
The SSI will be used to determine structure, motions, and radiative
properties of the atmosphere of Jupiter. It will measure wind
profiles by tracking how fast clouds move at various altitudes.
Radiative properties of the atmosphere, which are important for
understanding energy management, will be determined by
measuring the scattering of light from specific features at various
wavelengths and at various angles of illumination. Observations of
auroral phenomena will be correlated with fields and particles
measurements done with other instruments.
The SSI is an 800- by 800-pixel solid-state camera consisting of an
array of silicon sensors called a "charge-coupled device" (CCD). The
optical portion of the camera is built as a Cassegrain (reflecting)
telescope. Light is collected by the primary mirror and directed to a
smaller secondary mirror that channels it through a hole in the
center of the primary mirror and onto the CCD. The CCD sensor is
shielded from radiation, a particular problem within the harsh Jovian
magnetosphere. The shielding is accomplished by means of a
1-centimeter-thick layer of tantalum that surrounds the CCD except, of
course, where the light enters the system.
An eight-position filter wheel is used to obtain images of scenes
through different filters. The images may then be combined
electronically on Earth to produce color images.
The spectral response of the SSI ranges from about 0.4 to 1.1
micrometers. (A micrometer is one millionth of a meter.) Visible
light has a wavelength covering the band of 0.4 to 0.7 micrometers.
The SSI weighs 29.7-kilograms and consumes, on average, 15 watts
of power.
SSI DESCRIPTION
30K
- The SSI is a catadioptric telescope with a 150-cm focal length (f/8.5).
- The SSI telescope has a field of view of 8.1 mrad and an angular resolution
of 10.16
urad/pixel.
- The SSI optical system is a modified Voyager narrow-angle telescope flight spare.
- The SSI camera shutter and eight-position filter wheel are inherited from Voyager.
DESIGN DETAILS
- The filter wheel has eight filters, of the interference type, centered at 611, 404, 559,
671, 734, 756, 887, and 986 nm.
- The SSI detector is a virtual phase, buried channel, thick, frontside illuminated,
800 x 800 line CCD.
- The SSI has a pre-flash system to eliminate residual images after each exposure. This
system bathes the CCD in near-infrared light (~930 nm) several times and reads the
CCD several times at high speed.
- The SSI has five transmissive elements plus two mirrors.
- All SSI optical surfaces have anti-reflective coatings with a transmission factor of
62% at 576 nm.
- The front optical element is coated to minimize radiated heat loss and heaters provide
thermal stability across the front aperture and within the body of the telescope.
- The long wavelength sensitivity limit (~1100 nm) is set by the detector and
the short
wavelength limit (~375 nm) by the anti-reflection coatings.
INSTRUMENT PARAMETERS
- Instrument Mass: 28 Kg
- Power Consumption:
- Microprocessor: 1802 type
- ROM/RAM: 3 Kb/3 Kb
- Fields of View: 8.1 mrad circular
- Size: 90 x 25 x 30 cm
- Thermal Range:'
- Operating
- Electronics: -10 to 18 Deg. C
- Rear Optics: -10 to 18 Deg. C
- Detector: -113 to -107 Deg. C
- Non-operating
- Electronics -10 to 30 Deg. C
- Rear Optics -10 to 30 Deg. C
- Detector -130 to 50 Deg. C
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